The Arabidopsis-accelerated cell death gene ACD2 encodes red chlorophyll catabolite reductase and suppresses the spread of disease symptoms

Accelerated cell death 2 (acd2) mutants of Arabidopsis have spontaneous spreading cell death lesions and constitutive activation of defenses in the absence of pathogen infection. Lesion formation in acd2 plants can be triggered by the bacterial toxin coronatine through a light-dependent process. Coronatine-triggered and spontaneous lesion spreading in acd2 plants also requires protein translation, indicating that cell death occurs by an active process. We have cloned the ACD2 gene; its predicted product shows significant and extensive similarity to red chlorophyll catabolite reductase, which catalyzes one step in the breakdown of the porphyrin component of chlorophyll [Wuthrich, K. L., Bovet, L., Hunziger, P. E., Donnison, I. S. & Hortensteiner, S. (2000) Plant J. 21, 189-198]. Consistent with this, ACD2 protein contains a predicted chloroplast transit peptide, is processed in vivo, and purifies with the chloroplast fraction in subcellular fractionation experiments. At some stages of development, ACD2 protein also purifies with the mitochondrial fraction. We hypothesize that cell death in acd2 plants is caused by the accumulation of chlorophyll breakdown products. Such catabolites might be specific triggers for cell death or they might induce cellular damage through their ability to absorb light and emit electrons that generate free radicals. In response to infection by Pseudomonas syringae, transgenic plants expressing excess ACD2 protein show reduced disease symptoms but not reduced growth of bacteria. Thus, breakdown products of chlorophyll may act to amplify the symptoms of disease, including cell death and yellowing. We suggest that economically important plants overexpressing ACD2 might also show increased tolerance to pathogens and might be useful for increasing crop yields.